The invention relates to a device for the continuous measurement of the thickness of layers which are deposited on a substrate at low pressure. A first light conductor the light from a light source to the growing layer and a second light conductor guides the light reflected on the interface between substrate and growing layer and the light reflected on the interface between the growing layer and the surrounding atmosphere to a detector. The detector measures layer thicknesses and indicates variations in the intensity of the light as a result of interference between the two reflected light rays which varies with the optical path length in the growing layer.
In Philips Technical Review 43, pp. 58-60, Jan. 1987 a method is described for measuring the thickness of an infrared-reflecting layer during the deposition on the inside of the outer glass envelope of low-pressure sodium lamps. This layer consists of In.sub.2 O.sub.3 :Sn deposition the layer takes place at approximately 500.degree. C.
The known measuring instrument consists of two light conductors having an outside diameter of 2.5 mm and a core diameter of 1.9 mm. Both conductors are provided (at their cold end) with a coupling member to which glass fibre bundles can be connected; they constitute the connection to a light source and a detector, respectively. The light of a light source is guided to the measuring site by one conductor. The beam emanating from the warm end, after reflection at the layer to be measured, is received by the other conductor and guided to the detector. In order to direct the optical axis of the two (parallel) conductors to the same measuring point, the end faces of the conductors are ground and/or polished at a certain angle to the longitudinal axis.
The principle of the measurement is based on interference. Since the light beam reflects at the two sides of the forming infrared reflection layer, two reflected beams are formed which interfere with each other as a result of a difference in optical path length. When the thickness of the layer increases the detector will consequently observe variations in the intensity of the incident light. From this the thickness of the layer can be monitored at any instant by a processing equipment coupled to the detector. The measuring method may be used to control the deposition process automatically.
The measuring instrument described cannot be used as such for measuring the layer thicknesses of layers which are deposited reactively at low pressure on the outside of a product or on a surface of complex shape for example, by means of a low-pressure CVD process. The use of this type of processes is increasing in technology because herewith a large number of products can be coated simultaneously and in a reproducible manner without the products having to perform complicated movements, for example, in physical vapour deposition processes and the shape is a problem.